A method for electrolyzing an alkali metal chloride by an ion exchange membrane method is known wherein an aqueous alkali metal chloride solution is electrolyzed by using a fluorinated cation exchange membrane as a diaphragm to produce an alkali metal hydroxide and chlorine. In order to maintain a good operation performance for a long time in an electrolytic soda process by an ion exchange membrane method, it is important to adjust and maintain the catholyte concentration in the cathode compartment within a prescribed range and uniformly.
A common method for adjusting the concentration is to supply water into the cathode compartment. However, this method has the following problem. Firstly, if the liquid circulation in the cathode compartment is inadequate, the catholyte concentration decreases in the vicinity where water is supplied, and inversely, the catholyte concentration tends to be high at a portion where dilution scarcely takes place. As a result, the current efficiency decreases. Further, if supply of water to the cathode compartment stops by some trouble, the catholyte concentration rapidly increases, and the current efficiency substantially decreases. In such a case, once the current efficiency decreases, it is difficult to return it to the initial level, and change of the membrane will be required.
Some means have been proposed to improve the liquid circulation in the cathode compartment in order to make the catholyte concentration uniform. They are, for example, a means wherein a conductive spacer is disposed between an electrode plate and an electrode sheet to form a down flow path for the electrolyte (JP-A-61-19789), a means having a cylindrical current distributing member attached to form a down flow path for the electrolyte (JP-A-63-11686), a means having a cylindrical internal circulation duct which will be a circulation flow path for the electrolyte (JP-A-4-289184), a means having an apparatus to uniformly distribute the electrolyte to anode and cathode compartments (JP-A-4-289185) and others (JP-A-4-289186, JP-A-4-350189, and JP-A-4-350190).